Grease composition

ABSTRACT

A grease composition includes at least one mineral-based or synthetic-based oil, individually or in a mixture; at least one fatty acid simple or complex metal soap as thickener; at least one molybdenum dithiocarbamate; and graphite. The grease can be used for constant velocity joints of a motor vehicle transmission.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Phase Entry of International Application No. PCT/IB2011/055621, filed on Dec. 12, 2011, which claims priority to French Patent Application Serial No. 1060441, filed on Dec. 13, 2010, both of which are incorporated by reference herein.

BACKGROUND AND SUMMARY

The present invention relates to grease compositions which can be used in constant velocity joints of drive trains of motor vehicles, or of other gear mechanisms for which greases with a low friction coefficient are sought.

A transmission or mechanical coupling joint is a mechanical system consisting of several parts which are moveable relatively to each other, or deformable, which allows mutual driving of two rotating parts, the axes of rotation of which occupy variable relative positions during operation. In other words, it is a link which allows transmission of the rotation of one axis to another axis which is moveable relatively to the first. A transmission joint is said to be a constant velocity joint if, at any instant, the speeds of rotation of both shafts are equal.

The movements inside constant velocity joints are complex, with a combination of rolling, sliding and rotations. Wear on the contact surfaces of the components occurs therein, but also significant frictional forces between the surfaces. The wear may result in a failure of the joints and the frictional forces may cause noise, vibrations and jolts in the drive train. Thus, the greases used in constant velocity joints should have an anti-wear effect and preferentially a low friction coefficient in order to reduce or prevent noises, vibrations and jolts.

Various known additives contribute to reducing wear and/or friction. Thus, known greases for constant velocity joints frequently contain anti-wear additives, which for example are phosphorus-containing or phosphorus- and sulfur-containing compounds, and friction modifiers, for example organic compounds containing molybdenum, which may have effects on either one of these properties, or even both of them. The use of solid lubricants, as friction modifiers, such as molybdenum bisulfide (MoS₂) or tungsten bisulfide (MoS₂) or graphite is also known.

Application FR 2 723 747 discloses high-temperature greases for constant velocity joints comprising mineral and/or synthetic base oils, polyurea thickeners and MoS₂ as a solid lubricant, as well as graphite and at least one organic molybdenum compound, preferentially molybdenum dithiocarbamate. The solid lubricants graphite, MoDTC, PTFE, make it possible to reduce the content of expensive solid lubricant MoS₂, but without however replacing it completely.

Solid lubricants of the MoS₂ type have a high cost and introduce a high metal content in the formulations, which is not desirable for environmental reasons. Nothing in this application suggests the lowering of the metal (Mo) contents of the grease by totally substituting other solid lubricants for the MoS₂. The molybdenum content of the disclosed compositions is of at least of the order of 5,000 ppm. No mention is made of specific molybdenum and graphite contents allowing optimization of the performances of the greases as regards wear and/or friction. Nothing in this application either suggests substitution of lithium soaps for polyureas as thickeners.

The greases thickened with polyureas are technically more complicated to manufacture, in particular because the components used in their manufacture, such as isocyanates and amines, are highly toxic and not very stable upon storage. The precautions which have to be taken, make the manufacturing methods more complex and more expensive. The availability of the raw materials is also lower as compared with those of greases thickened with metal soaps, in particular with lithium and lithium complex. The polyureas are superior in thermal resistance but their thixotropic nature also leads to problems of destructuration under mechanical stress and hardening upon storage. Therefore, technically easier and more economical greases for constant velocity joints will be preferred for formulating greases from non-toxic and non-hazardous materials using fatty acid metal soaps as thickeners.

Certain commercial greases for constant velocity joints are for example formulated from mineral and/or synthetic base oils, from lithium or lithium complex thickeners, and from molybdenum bisulfide MoS₂, in an amount of about 3% by mass, which represents a Mo content of about 18,000 ppm. Below this MoS₂ content, the wear and friction performances are insufficient.

Patent EP 0 708 172 describes a grease with a low friction coefficient for constant velocity joints comprising a base oil, a simple or complex lithium soap thickener, one or several organic components containing molybdenum, of the MoDTC or the MoDTP type, at least one zinc dithiosphosphate, an phosphorus- and sulfur-containing extreme pressure agent free of metal, calcium salt of oxidized waxes, calcium salt of petroleum sulfonate or calcium salt of aromatic alkylsulfonates. The disclosed composition examples either have a high molybdenum content (about 8,500 ppm) and/or a high phosphorus content (about 2,000 ppm of phosphorus), which leads to environmental problems. No mention is made of the possibility of partly substituting the compounds comprising molybdenum and/or phosphorus with other additives for lowering the metal content of the compositions, or of an optimum molybdenum content allowing preservation or improvement of the properties of these greases, notably as regards wear and friction.

The publication “Effect of graphite on friction and wear characteristics of molybdenum dithiocarbamate”, Y. Yamamoto et al., Tribology Letters, Vol 17, N° 1, July 2004, discloses the booster effect of graphite on the performances of MoDTC, in a specific lubricant oil, squalane. This improvement is especially appreciable in the presence of a succinimide dispersant. This document does not relate to the field of greases. This study is limited to a single natural lubricant oil and does not deal with the field of greases. No specific combination of a base oil, of graphite and of MoDTC with other components necessary for the formulation of greases, in particular thickeners, is disclosed. It is therefore absolutely impossible to conclude from this publication whether the booster effect of graphite on the friction and wear performances of MoDTC will be reproduced in a grease comprising a particular thickener. No optimization of the molybdenum and graphite contents is carried out.

Application WO 2007/085643 discloses grease compositions for constant velocity joints, thickened with polyureas, comprising from 0.1 to 5% by weight of WS₂, and from 0.1 to 5% by weight of zinc dithiophosphate and/or molybdenum dithiocarbamate. This application also discloses the possibility of using as an additive in the greases, graphite or MoS₂. However, the use of graphite (or MoS₂) in combination with MoDTC is not recommended, since this combination, according to this application, has a detrimental effect on the anti-wear properties and on the friction coefficient of the greases.

Therefore, there exists a need for greases for constant velocity joints, containing non-toxic, available and inexpensive raw materials, easy to make, having reduced molybdenum (Mo) and phosphorus (P) content and having adequate anti-wear properties and friction properties (low friction coefficient) for this application. The applicant has shown that greases for constant velocity joints thickened with fatty acid metal soaps and comprising graphite combined with molybdenum dithiocarbamate give the possibility of meeting the need discussed above.

These greases according to the invention have improved anti-wear properties as compared with known greases for constant velocity joints based on lithium soap. The friction properties are equivalent to those of known greases. These greases may moreover attain these performances with lower molybdenum and optionally lower phosphorus content than the greases of the prior art.

The present relation relates to grease compositions comprising:

a. one or several mineral or synthetic base oils, either alone or as a mixture

b. one or several simple or complex metal salts of fatty acids as a thickener,

c. at least one molybdenum dithiocarbamate,

d. graphite.

Preferentially, the grease compositions according to the invention have a molybdenum content comprised between 100 and 2,800 ppm preferentially comprised between 1,500 and 2,500 ppm, preferentially between 1,700 and 2,300, preferentially between 2,000 and 2,200 ppm. Preferentially, the grease compositions according to the invention comprise between 0.5 and 3.0% by mass of graphite, preferentially between 0.7 and 2.0%. More preferably, the grease compositions according to the invention have a Mo/[graphite] ratio, between the molybdenum content in ppm and the percentage by mass of graphite in said compositions, comprised between 1,250 and 1,550, preferentially between 1,300 and 1,500.

According to an embodiment, the grease compositions according to the invention comprise phosphorus, at a content of less than 1,500 ppm, preferentially less than 1,200 ppm. According to an embodiment, the grease compositions according to the invention comprise zinc, at a content of less than 1,500 ppm. According to a preferred embodiment, the grease compositions according to the invention are free of solid lubricant MoS₂.

According to another preferred embodiment, the grease compositions according to the invention are free of any polytetrafluoroethylene solid lubricant. Preferentially, the grease compositions according to the invention exclusively comprise one or several simple or complex metal salts of fatty acids as a thickener. Preferentially, the grease compositions according to the invention comprise at least one simple or complex lithium, sodium, calcium, barium, or titanium soap, either alone or as a mixture, as a thickener. Still more preferentially, the grease compositions according to the invention comprise one or several simple or complex lithium soaps as a thickener.

According to an embodiment, the grease compositions according to the invention comprise a mineral base oil (a) and a synthetic base oil (a), preferentially selected from polyalphaolefins. According to a preferred embodiment, the grease composition according to the invention further comprise a polymer (e) preferentially selected from polyisobutenes, olefin copolymers, polymethacrylates, polyalphaolefins, preferentially polyisobutenes. The grease compositions according to the invention may further comprise one or several anti-wear and/or extreme pressure additives containing phosphorus and sulfur (f), preferentially selected from zinc dithiophosphates.

Preferentially, the grease compositions according to the invention have a phosphorus content comprised between 300 and 1,200 ppm, preferentially between 400 and 1,000 ppm, preferentially between 500 and 900 ppm. Preferentially, the compositions according to the invention have a zinc content comprised between 500 and 1,400 ppm, preferentially between 600 and 1,300 ppm, preferentially between 800 and 1,000 ppm.

According to an embodiment, the grease compositions according to the invention comprise:

from 70 to 94% of base oils (a),

from 5 to 25% of thickener (b),

from 1 to 5% of mixture of MoDTC (c) and graphite (d).

According to another embodiment, the grease compositions according to the invention comprise:

-   -   from 70 to 94% of base oils (a),     -   from 5 to 25% of thickener (b),     -   from 1 to 5% of mixture of MoDTC (c) and graphite (d),     -   the possible remainder consisting of one or several polymers (e)         selected from polyisobutenes, olefin copolymers,         polymethacrylates, polyalphaolefins, preferentially         polyisobutenes.

According to a preferred embodiment, the grease compositions according to the invention comprise:

-   -   from 70 to 93.9% of base oils (a),     -   from 5 to 23.5% of thickener (b),     -   from 1 to 5% of mixture of MoDTC (c) and graphite (d),     -   from 0.1 to 1.5% of one or several anti-wear and/or extreme         pressure additives containing phosphorus and sulfur (f),         preferentially selected from zinc dithiophosphates.

According to another preferred embodiment, the grease compositions according to the invention comprise:

-   -   from 70 to 93.9% of base oils (a),     -   from 5 to 23.5% of thickener (b),     -   from 1 to 5% of mixture of MoDTC (c) and graphite (d),     -   from 0.1 to 1.5% of one or several anti-wear additives either         containing phosphorus or containing phosphorus and sulfur (f),         preferentially selected from zinc dithiophosphates.     -   The possible remainder consisting of one or several polymers (e)         selected from polyisobutenes, olefin copolymers,         polymethacrylates, polyalphaolefins, preferentially         polyisobutenes.

The present invention also relates to the use of grease compositions such as those described above as greases for constant velocity joints of transmissions of automobile vehicles. The present invention also relates to one of the constant velocity joints filled with grease compositions as described above.

DETAILED DESCRIPTION

Lubricant Base Oils (a)

The or the other base oil(s) used in the composition according to the present invention can be mineral or synthetic oils originating from the Groups I to V according to classes defined in the API (American Petroleum Institute) classification. The mineral base oils according to the invention include all types of bases obtained by atmospheric and vacuum distillation of crude oil, followed by refining operations such as extraction with a solvent, deasphalting, dewaxing with a solvent, hydrotreatment, hydrocracking and hydroisomerization, hydrofinishing. The base oils of grease compositions according to the present invention can also be synthetic oils such as certain esters, silicones, polyalkylene glycols, polybutene, polyalphaolefins (PAO), alkylbenzene, alkyl naphthalene. The base oils can also be oils of natural origin, for example alcohol and carboxylic acid esters which may be obtained from natural resources such as sunflower, rapeseed, palm oil, . . . .

Preferentially, in the compositions according to the invention, synthetic oils of the polyalphaolefin (PAO) type are present in a combination with mineral oils. The polyalphaolefins are for example obtained from monomers having from 4 to 32 carbon atoms (for example octane, decene). Their weight average molecular mass is typically comprised between 250 and 3,000.

The mixture of base oils is set so that its viscosity at 40° C. according to the ASTM D445 standard is comprised between 30 and 140 cSt, preferentially between 50 and 100 cSt. For this purpose, a wide range of light polyalphaolefins such as for example PAO 6 (31 cSt at 40° C.), PAO 8 (48 cSt at 40° C.), or heavy polyalphaolefins, such as PAO 40 (400 cSt at 40° C.), or PAO 100 (1,000 cSt at 40° C.) may be used.

Thickeners (b)

The greases according to the invention are thickened with fatty acid metal soaps. Fatty acid metal soaps may be prepared separately, or in situ during the making of the grease (in the latter case, the fatty acid(s) is(are) dissolved in the base oil, and then the suitable metal hydroxide is added). These thickeners are products currently used in the field of greases, which are easily available and inexpensive. They exhibit the best technical compromise by combining both good mechanical properties, good thermal resistance and good resistance to water.

Fatty acids with a long chain typically comprising from 10 to 28 carbon atoms, saturated or unsaturated, eventually hydroxylated, are preferentially used. The long chain fatty acids (typically comprising from 10 to 28 carbon atoms) are for example capric, lauric, myristic, palmitic, stearic, arachidic, behenic, oleic, linoleic, erucic acids and their hydroxylated derivatives. 12-hydroxystearic acid is the most well-known derivative of this category, and is preferred. These long chain fatty acids generally stem from plant oils, for example palm, castor, rapeseed, sunflower oil, . . . or from animal fats (tallow, whale oil . . . ). So-called simple soaps may be formed by using one or several long chain fatty acids. It is also possible to form so-called complex soaps by using one or several long chain fatty acids in combination with one or several carboxylic acids with a short hydrocarbon chain comprising at most 8 carbon atoms.

The saponification agent used for producing the soap may be a metal compound of lithium, sodium, calcium, barium, titanium, aluminum, preferentially lithium and calcium, and preferably a hydroxide, oxide or carbonate of these metals. Metal soaps, in particular of lithium, sodium, calcium, barium, titanium, form a fibrous structure except for the simple or complex aluminum soaps which have a spherical gel structure.

One or several metal compounds either having the same metal cation or not can be used in the greases according to the invention. Thus it is possible to associate lithium soaps, combined with calcium soaps in a lesser proportion. This has the advantage of improving the water resistance of the greases.

Metal soaps are used at contents of the order of 5 to 20% by weight, preferentially from 8 to 15% by weight, preferentially from 10 to 12%, typically 11% by weight in the greases according to the invention. The amount of metal soap(s) is generally adjusted so as to obtain greases of grade 00, grade 0, grade 1 or grade 2 according to the NLGI classification. Preferentially, the greases according to the invention in majority contain fatty acid metal soaps as a thickener. By this is meant that the simple or complex fatty acid metal soaps together represent the highest percentages by weight in the greases according to the invention, as compared with the percentage by weight of the other thickening material.

Preferentially, the greases according to the invention contain in majority simple or complex lithium soaps as a thickener. By this is meant that the simple or complex lithium soaps together represent the highest percentages by weight in the greases according to the invention, as compared with the percentage by weight of the other thickening materials. Preferentially, the amount of the simple or complex fatty acid metal soap(s) forms at least 50%, even more preferentially at least 80% by weight based on the total weight of thickening materials, in the grease compositions according to the invention. Preferentially, the amount of simple or complex lithium soaps forms at least 50%, even more preferentially at least 80% by weight based on the total weight of thickening materials, in the grease compositions according to the invention.

According to an embodiment, the greases according to the invention can contain simple or complex metal soaps of fatty acids as the main thickener, and smaller quantities of other thickeners, such as polyureas, or inorganic thickeners of the bentonite or aluminosilicates type. Preferentially, the greases according to the invention are free of polyurea-type thickeners, which are technically more complicated to manufacture, in particular because the components used in their manufacture, such as isocyanates and amines, are very toxic and not very stable upon storage. Even more preferentially, the greases according to the invention exclusively contain simple or complex fatty acid metal soaps as a thickener. Still more preferentially, the greases according to the invention exclusively contain simple or complex fatty acid metal soaps of lithium as a thickener.

Molybdenum Dithiocarbamate and Graphite

The grease compositions according to the invention contain graphite and molybdenum dithiocarbamate, which gives them improved anti-wear properties as compared with known commercial greases. These properties may be reached with a lower molybdenum content than the one of known greases.

Preferentially, the molybdenum content of the greases according to the invention is comprised between a 1,000 and 2,800 ppm. Preferentially, the Mo/[graphite] ratio, between the molybdenum content in ppm and the percentage by mass of graphite in said compositions, is comprised between 1,250 and 1,550. Actually it is seen that when these compounds are present in the aforementioned proportions, the grease compositions also have very good anti-wear and friction properties, equivalent to those of known commercial greases. These good properties are moreover reached with a molybdenum content, and optionally a content of other metal elements such as zinc, or a phosphorus element content, of less than those of known greases.

These performances can be reached without it being necessary to add to the grease compositions according to the invention, solid lubricant additives such as MoS₂ or polytetrafluoroethylene (PTFE) powders. The possible contents of phosphorus-containing additives, notably anti-wear additives, can also be low.

Molybdenum Dithiocarbamate (DTCMo) (c)

The compositions according to the invention contain molybdenum dithiocarbamates, friction modifier additives which are well known to one skilled in the art. These molybdenum dithiocarbamate organometallic friction modifiers can for example be molybdenum dialkyldithiocarbamates corresponding to the formula (I):

wherein X₁, X₂, X₃, X₄ are alkyl chains, preferentially including from 2 to 13 carbon atoms, preferentially from 2 to 6 carbon atoms.

The amount of MoDTC of the compositions according to the invention is adjusted so that their molybdenum content is comprised between 1,000 and 2,800 ppm, preferentially between 1,500 and 2,500 ppm, preferentially between 1,700 and 2,300, preferentially between 2,000 and 2,200 ppm. This content can be measured according to usual techniques: plasma, atomic absorption, X-ray fluorescence.

It is not desirable to use MoDTC as a single additive (anti-wear) in a grease composition. Indeed, it is known that MoDTC requires a minimum activation temperature and the presence of other additives (anti-wear, extreme pressure additives) in order to be efficient. Greases comprising MoDTC without any other anti-wear and/or extreme pressure additive would not be efficient, in particular at a low temperature. In the greases of the prior art, MoDTC is thus combined with phosphorus-containing additives, for example dithiophosphates.

In the greases according to the invention, MoDTC is combined with graphite, which allows production of greases having very good properties as regards wear and friction, with a low molybdenum and phosphorus level. When the Mo content is too low, the anti-wear properties (notably under a high load), will be insufficient for applications to constant velocity joints. When the MoDTC content is too high (to the detriment of graphite), it is seen that the friction performances of the greases are degraded. Moreover, it is not desirable for environmental reasons, to have too high contents of the Mo element in the greases. Beyond the Mo thresholds mentioned above, no significant gain as regards wear is observed.

Graphite (d)

The grease compositions according to the invention contain graphite. The presence of graphite, known for its properties as a solid lubricant, allows reduction in the content of friction modifier and anti-wear additives containing molybdenum (and optionally phosphorus), without providing any metal element, and while maintaining wear and friction properties suitable for the application to constant velocity joints. Entire substitution of these molybdenum additives with graphite is however not desirable, since graphite is not very efficient under a strong load and problems of wear may occur in such greases. Also an increase in the wear is seen when the amount of graphite is too large to the detriment of molybdenum.

Preferentially, the graphite used in the compositions according to the invention is a micrometric size powder with sizes of particles comprised about between 1 and 15 μm, and for example a size distribution characterized by a diameter D50 comprised between 3 and 8 μm, preferentially between 5 and 7 μm. The grease compositions according to the invention preferentially comprise between 0.5 and 3.0% by mass of graphite, preferentially between 0.7 and 2.0%, preferentially between 0.75 and 1.7%, preferentially between 1.0 and 1.5% by mass of graphite.

Compounds Containing Phosphorus and Sulfur (f)

The greases according to the invention optionally contain anti-wear and extreme pressure additives containing phosphorus and sulfur, commonly used in the formulation of greases and lubricants. These are for example and in a non-limiting way, thiophosphoric acid, thiophorous acid, esters of these acids, their salts and dithiophosphates, particularly zinc dithiophosphates.

The zinc dithiophosphates of formula (II) are in particular preferred:

wherein R₁, R₂, R₃, R₄ are, independently of each other, linear or branched alkyl groups comprising from 1 to 24, preferentially from 3 to 14 carbon atoms or optionally substituted aryl groups including from 6 to 30, preferentially from 8 to 18 carbon atoms.

These different compounds may be used alone or as a mixture in the grease compositions according to the invention. Their percentage by mass in the compositions according to the invention is preferably comprised between 0.5 and 5% by weight, preferentially between 0.7 and 2% by weight, or further between 0.8 and 1.5% by weight based on the total weight of the composition. Their amount will be adjusted so as to observe the limiting contents of the molybdenum and phosphorus elements in the compositions according to the invention.

The molybdenum (Mo) content of the compositions according to the invention is preferentially comprised between 1,000 and 2,800 ppm, preferentially between 1,500 and 2,500 ppm, preferentially between 1,700 and 2,300, preferentially between 2,000 and 2,200 ppm. The phosphorus (P) content of the compositions according to the invention is preferentially less than 1,500 ppm, preferentially less than 1,200 ppm, preferentially comprised between 300 and 1,200 ppm, preferentially between 400 and 1,000 ppm, preferentially between 500 and 900 ppm. This content may be measured according to customary techniques: plasma, atomic absorption techniques. Preferentially, when the compositions comprise zinc dithiophosphate, the zinc content of the compositions according to the invention is less than 1,500 ppm, preferentially comprised between 500 and 1,400 ppm, preferentially between 600 and 1,300 ppm, preferentially between 700 and, 1200 ppm, preferentially between 800 and 1,000 ppm.

The lubricant compositions according to the present invention may also contain anti-wear and extreme pressure additives containing phosphorus, such as for example alkyl phosphates or alkyl phosphonates, phosphoric acid, phosphorous acid, mono-, di- and tri-esters of phosphorous acid and of phosphoric acid, and their salts. The lubricant composition according to the present invention can also contain anti-wear and extreme pressure additives containing sulfur, such as dithiocarbamates, thiadiazoles and benzothiazoles, sulfur-containing olefins.

Other Additives

The greases according to the invention can also contain any type of additives suitable for their use, for example antioxidants, such aminated or phenolic antioxidants, antirust agents which may be oxygenated compounds such as esters, copper passivating agents. These different compounds are generally present at contents of less than 1%, or further than 0.5% by mass in the greases.

The greases according to the invention may also contain polymers (e), for example polyolefins, polyisobutene (FIB), polyethylene, polypropylene, heavy PAOs, olefin copolymers (OCP) for example hydrogenated diene-styrene, polymethacrylates (PMA), in contents generally comprised between 1 and 35%. For example PIBs of a molar mass comprised between 15,000 and 25,000 Daltons will be used at contents generally comprised between 2 and 15% by mass or between 5 and 10% by mass.

These polymers are used for improving the cohesiveness of the greases, which thus better resist centrifugation. These polymers also cause better adhesion of the grease to surfaces, and increase the viscosity of the base oil fraction, therefore the thickness of the oil film between the parts subject to friction.

Method for Preparing the Greases

The greases according to the invention are preferentially made by forming the metal soap in situ. One or several fatty acids are dissolved in fraction of the base oil or of the base oil mixture at room temperature. This fraction is generally of the order of 50% of the total amount of oil contained in the final grease. The fatty acids can be long acids, comprising from 14 to 28 carbon atoms, in order to form simple soaps, optionally combined with short fatty acids comprising from 6 to 12 carbon atoms, in order to form complex soaps.

The metal compounds preferentially of the metal hydroxide type are added at a temperature of about 60 to 80° C. It is thus possible to add a single type of metal or combine several metals. The preferred metal of the compositions according to the invention is lithium, optionally combined in a lesser proportion with calcium.

The reaction for saponification of the fatty acids by the metal compound(s) is left to take place at a temperature from about 100 to 110° C. The water formed is then evaporated by baking the mixture at a temperature of about 200° C. The grease is then cooled by the remaining fraction of base oil. The additives are then incorporated at about 80° C. Kneading is then carried out for sufficient time in order to obtain a homogeneous grease composition.

Grade of the Greases

The consistency of a grease measures its hardness or its fluidity at rest. It is quantified by the penetration depth of a cone with given dimensions and mass. The grease is subject beforehand to kneading. The conditions for measuring the consistency of a grease are defined by the ASTM D217 standard.

According to their consistency, the greases are divided in 9 NLGI (National Lubricating Grease Institute) classes or grades commonly used in the field of greases. These grades are indicated in the table below. The greases according to the invention are preferentially greases with consistency comprised between 265 and 430, preferentially between 265 and 385, preferentially between 265 and 340 tenths of a millimeter according to ASTM D217. Preferentially, they are of NLGI grade 00, 0, 1 or 2, i.e. their consistency is respectively comprised between 400 and 430, or 335 and 385 or 310 and 340, or 265 and 295 tenths of a millimeter according to ASTM D217.

Grade of the greases Consistency according to ASTM D 217 (tenths of a NLGI grade millimeter) 000 445-475 00 400-430 0 335-385 1 310-340 2 265-295 3 220-250 4 175-205 5 130-160 6 85-15

EXAMPLES 1. Preparation of Grease Compositions

Grease compositions containing various additives, notably graphite and molybdenum dithiocarbamate are prepared from a same base grease comprising mineral and synthetic base oils, thickened with a simple lithium soap prepared from lithine (LiOH.H₂O), and 12-hydroxystearic acid. The composition of the base grease is indicated in Table 1 below. The term of “base grease” commonly designates for one skilled in the art, a grease composition only containing base oils and thickeners, but no additive.

TABLE 1 composition of the base grease Compound % by mass Mineral oils 80.85 Synthetic oils (PAO) 9.00 Simple lithium soap 10.15

From this base grease, various additives are added: a polymer (FIB) for improving cohesiveness and adhesion, an anti-wear agent (DTPZn), and various solid lubricants: MoS₂, graphite, MoDTC.

-   -   The graphite used is a micrometric powder of diameter D50=6 μm     -   The MoDTC used is molybdenum di-n-butyldithiocarbamate,         containing 28% by mass of molybdenum     -   The MoS₂ used is a micrometric powder consisting of particles         with a size comprising 0.5 and 8 μm, with D50 of about 2 μm.

All the prepared greases are of grade 2 according to the NLGI classification. Grease A is a commercial grease for constant velocity joints, the greases E, I and J are according to the invention. The mass compositions of the greases are given in Table 2, as well as their wear and friction properties. The wear properties were evaluated by measuring the wear diameter (in millimeters) after the 4 ball wear test according to the ASTM D2266 standard (1 hour, 40 kg load, 75° C.). According to the STL S71 3100 specification, the greases for constant velocity joints should lead to a wear diameter of less than 50 mm.

The friction properties were evaluated by measuring the average friction coefficient on a Cameron Plint cylinder-plane tribometer, under the conditions hereafter:

-   -   Mobile pin: SKF100C6 cylinder (diameter 7 mm×length 7 mm)     -   Fixed plane: XC38 without THT, circular polishing (Ra 0.8 μm)     -   Travel: 2.5 mm     -   Frequency: 5 levels between 2.5 and 40 Hz, +1/20 reducer     -   Load: 3 levels, 10, 50, 110 N     -   Duration: 20 sequences, a total of 1,786 seconds.         -   16 plateaus stabilized for 80 seconds with the average             friction calculated over the last 40 seconds         -   2 rapid acquisitions over 3 instantaneous friction cycles         -   2 functional steps (initial run-in period+change in             temperature)     -   Temperature: 2 phases: low temperature phase (50° C.) and then a         high temperature phase (100° C.)     -   Grease volume: 1 g distributed as a thin and continuous film         before the test

All the greases containing MoDTC combined with graphite have a molybdenum content much lower than that of the commercial grease A, for improved wear performances. The greases E and I have the additional advantage of having low friction coefficients, of the order of magnitude of those of the commercial grease A.

TABLE 2 composition and properties of the greases A B C D E F G H I J % base grease % 89.80 90.54 90.54 90.55 90.55 90.55 90.55 90.55 91.67 89.80 by mass MoS₂ % 3.00 MoDTC % 1.13 0.97 0.90 0.75 0.64 0.56 0.45 0.38 1.00 Graphite % 1.13 1.29 1.35 1.50 1.61 1.69 1.80 0.75 2.00 DTPZn % 1.20 1.20 1.20 1.20 1.20 1.20 1.20 1.20 1.20 1.20 Polymer (PIB) % 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 Mo content (ppm) 18000 3164 2716 2520 2100 1792 1568 1260 1064 2800 P content (ppm) 1140 1140 1140 1150 1140 1140 1140 1140 1140 1140 Zn content (ppm) 1272 1272 1272 1272 1272 1272 1272 1272 1272 1272 % of graphite/% of 0 1.00 1.33 1.50 2.00 2.52 3.02 4.00 1.97 2.00 MoDTC % of MoDTC/% of 1.00 0.75 0.67 0.50 0.40 0.33 0.25 0.51 0.50 graphite Mo ppm/% of graphite 2800 2105 1867 1400 1113 928 700 1419 1400 4 wear balls ASTM 0.49 0.42 0.44 0.47 0.4 0.41 0.43 0.42 0.48 0.46 D2266 1 h/40 kg. 75° C. Cameron Plint 0.093 0.143 0.14 0.149 0.096 0.147 0.141 0.155 0.113 0.126 Average friction coefficient 

1. A grease composition comprising: a) one or several mineral or synthetic base oils, either alone or as a mixture; b) one or several simple or complex fatty acid metal soaps as a thickener; c) at least one molybdenum dithiocarbamate; and d) graphite. 2-22. (canceled)
 23. The grease composition according to claim 1, having a molybdenum content comprised between 1,000 and 2,800 ppm.
 24. The grease composition according to claim 1, comprising between 0.5 and 3.0% by mass of graphite.
 25. The grease composition according to claim 1, wherein the Mo/[graphite] ratio, between the molybdenum content in ppm and the mass percentage of graphite in the composition, is comprised between 1,250 and 1,550.
 26. The grease composition according to claim 1, free of MoS₂ solid lubricant.
 27. The grease composition according to claim 1, free of polytetrafluorethylene solid lubricant.
 28. The grease composition according to claim 1, exclusively comprising one or several simple or complex fatty acid metal soaps as a thickener.
 29. The grease composition according claim 1, comprising as a thickener, at least one simple or complex soap of lithium, sodium, calcium, barium, or titanium, either alone or as a mixture.
 30. The grease composition according to claim 1, comprising one or several simple or complex lithium soaps as a thickener.
 31. The grease composition according to claim 1, wherein the oils comprise the mineral base oil and the synthetic base oil.
 32. The grease composition according to claim 1, further comprising a polymer.
 33. The grease composition according to claim 1, further comprising one or several anti-wear and/or extreme pressure additives containing phosphorus and sulfur.
 34. The grease composition according to claim 33, wherein the phosphorous content is comprised between 300 and 1,200 ppm.
 35. The grease composition according to claim 33, wherein the anti-wear and/or extreme pressure additives containing phosphorus and sulfur is zinc dithiophosphates and wherein the zinc content is comprised between 500 and 1,400 ppm.
 36. The grease composition according to claim 1, comprising: from 70 to 94% of base oils (a); from 5 to 25% of thickener (b); and from 1 to 5% of the mixture of MoDTC (c) and graphite (d).
 37. The grease composition according to claim 32, comprising: from 70 to 94% of base oils (a); from 5 to 25% of thickener (b); from 1 to 5% of the mixture of MoDTC (c) and graphite (d); and the possible remainder consisting of one or several polymers selected from polyisobutenes, olefin copolymers, polymethacrylates, and polyalphaolefins.
 38. The grease composition according to claim 32, comprising: from 70 to 93.9% of base oils (a); from 5 to 23.5% of thickener (b); from 1 to 5% of the mixture of MoDTC (c) and graphite (d); and from 0.1 to 1.5% of one or several anti-wear and/or extreme pressure additives containing phosphorus and sulfur, selected from zinc dithiophosphates.
 39. The grease composition according to claim 32, comprising: from 70 to 93.9% of base oils (a); from 5 to 23.5% of thickener (b); from 1 to 5% of the mixture of MoDTC (c) and graphite (d); from 0.1 to 1.5% of one or several anti-wear and/or extreme pressure additives containing phosphorus or containing phosphorus and sulfur; and the possible remainder consisting of one or several polymers selected from polyisobutenes, olefin copolymers, polymethacrylates, and polyalphaolefins.
 40. A method for the lubrication of constant velocity joints of automobile vehicle transmissions with a grease composition, the method comprising contacting the grease composition with the constant velocity joints of automobile vehicle transmissions, the grease composition comprising: a) one or several mineral or synthetic base oils, either alone or as a mixture; b) one or several simple or complex fatty acid metal soaps as a thickener; c) at least one molybdenum dithiocarbamate; and d) graphite.
 41. A constant velocity joint filled with a grease comprising: a) one or several mineral or synthetic base oils, either alone or as a mixture; b) one or several simple or complex fatty acid metal soaps as a thickener; c) at least one molybdenum dithiocarbamate; and d) graphite. 